fix: attempt fix for compiler error on Linux

    xmap.c: In function ‘commandt_xmap’:
    xmap.c:18:9: error: ‘MAP_ANONYMOUS’ undeclared (first use in this function)
       18 |         MAP_ANONYMOUS | MAP_NORESERVE | MAP_PRIVATE,
          |         ^~~~~~~~~~~~~
    xmap.c:18:9: note: each undeclared identifier is reported only once for each function it appears in
    xmap.c:18:25: error: ‘MAP_NORESERVE’ undeclared (first use in this function)
       18 |         MAP_ANONYMOUS | MAP_NORESERVE | MAP_PRIVATE,
          |                         ^~~~~~~~~~~~~

GNU extensions, apparently, and when I was using Clang default target
(gnu17) I wasn't seeing this (and on Linux/GCC, not sure what the
default is, but it wasn't needed).

lua/wincent/commandt/lib/xmap.c

@@ -5,6 +5,8 @@
 
 #include "xmap.h"
 
+#define _GNU_SOURCE /* for MAP_ANONYMOUS, MAP_NORESERVE */
+
 #include <assert.h> /* for assert() */
 #include <stddef.h> /* for NULL */
 #include <stdlib.h> /* for abort() */